This invention relates to a cell-box-type noise barrier having a large magnitude of transmission loss and aims to prevent propagation of various forms of noise issuing from wheeled vehicles travelling at high speeds and from mechanical devices productive of high levels of sound and to a method for directly insulating noise from a noise source.
In recent years, noise pollution from such industrial sources as canneries and pressing shops and from such traffic sources as express ways and railroads has become so great as pose a serious social issue. To cope with this problem, engineering circles have been urged both to minimize the absolute number of noise sources and at the same time to enclose noise sources with noise barriers which are capable of absorbing the noises on their inner sides and of preventing the noises from being propagated beyond their outer sides.
The structures of such noise barriers make use of the working principles of acoustics.
The main target at which these structures are aimed resides in maximizing the transmission loss of noise (equal to 10 times the common logarithm of the ratio of the energy of received sound to the energy of transmitted sound, expressed in decibels, dB) at each frequency level within the audible range for the average person.
Generally in the case of a wall structure made of a unitary material (such as of a metal plate, for example), one can attempt to increase the magnitude of transmission loss by adding to the thickness of the wall. At a certain frequency level, however, the phenomenon of resonance occurs between the sound pressure distribution caused by the sound wave impinging upon the wall surface and the proper vibration distribution of the wall material, giving rise to the effect of coincidence. This effect brings about a decline in the magnitude of transmission loss within the audible range of frequence which is a great obstacle to the prevention of noise propagation. The phenomenon of decreased transmission loss due to resonance is inherent in a barrier structure composed of hollow cell-type blocks, although the magnitude of transmission loss itself is higher than that attainable with the wall structure made of a unitary material.
A primary object of the present invention is to provide a noise barrier which has a higher transmission loss than the wall structure made of a unitary material or the wall structure composed of hollow cell-type blocks.
Another object of the present invention is to provide a noise barrier which displays no decline in the magnitude of transmission loss due to the aforementioned phenomenon of resonance. A still further object of the present invention is to provide an extremely simple and effective method for directly insulating the noise from a noise source.